1. Field of the Invention
The present invention relates to a color image processing apparatus for executing reading and recording of color image signals.
2. Description of the Related Art
The construction of the prior art color image processing apparatus adapted for use in a color scanner, a color facsimile and so on will be described below with reference to FIG. 1.
As illustrated in FIG. 1, the prior art color image processing apparatus is constituted by an input image processing circuit A and an output image processing circuit B.
The input image processing circuit A includes a color image sensor 11, A/D converters 12 through 14, and shading correction circuits 15 through 17, and executes input processing of three color image signals consisting of R(Red), G(Green) and B(Blue) color signals.
The output image processing circuit B includes a color-transformation processing circuit 33, and an image format transform circuit 34, and executes output processing of the three-color image signals outputted from the input image processing circuit A.
The color image sensor 11 is formed by a contact type color image sensor (CIS) constituted, e.g., by either a plurality of charge coupled devices (CCD) or a plurality of photoelectric conversion devices, arranged one-dimensionally, respectively, for three colors, i.e., the R, G, and B colors (the RGB three colors).
The operation of the color image processing apparatus having the above-described construction will be provided hereinbelow.
The color image sensor 11 converts manuscript images into analogue color image signals of the RGB three colors, and outputs these signals. The respective RGB three-color analogue signals outputted from the color image sensor 11 are converted into corresponding digital multi-level signals by the A/D converters 12 through 14. Then, the multi-level signals are inputted into the respective shading correction circuits 15 through 17 so as to be subjected to correction of output distortion (shading) in the scanning direction of the color image sensor 11, and are outputted therefrom as corrected three-color image signals.
The outputted three-color image signals from the shading correction circuits 15 through 17 are processed in the color-transformation processing circuit 33 to receive a gradation correction and a color-coordinates transformation and thereafter, the signals are processed by the image-format transform circuit 34 to be subjected to a transform processing by which they are transformed into color image data according to a predetermined format such as the bitmap and the JPEG.
In the color image processing apparatus, for the purpose of achieving speed-up of reading of the color image signals, it is necessary to shorten the accumulation time in the color image sensor as well as to increase the speed of the transferring clock. In this connection, the drive system for the luminous source such as the CCD and CIS used for the color image sensor is increased in its driving speed owing to the technical advancement of recent years.
Nevertheless, when the luminance of the luminous source used for the color image sensor is constant, the amplitudes of the RGB three-color image signals might be reduced causing deterioration of the S/N ratio.
In order to prevent the S/N ratio from being deteriorated, it might be possible to adopt a method of increasing luminance of the luminous source. However, due to the physical property of the luminous elements constituting the luminous source, there is a limitation in the acquired luminance of the luminous source. Although an employment of a plurality of luminous sources to be mounted in the color image sensor might enable it to increase the luminance, the number of luminous sources that can be mounted in the sensor must be restricted from the viewpoint of dimensional requirement in constructing the color image processing apparatus.
Therefore, the deteriorated S/N ratio must be restored by image processing, in order to achieve the speed-up of the reading of color image signals. More specifically, it is necessary to implement a process of reducing any noise component contained in the color image signal.
However, if the processing of reducing the noise component is complicated prolonging the time necessary for implementing the process, the aimed speed-up of reading of the color image signals will be prevented. To this end, there are many requests to realize a method of simply and effectively achieving the processing of reducing the noise component contained in the color image signals.
As one simple process that is capable of reducing the noise component, a process of moving average can be taken up, in which with respect to any notable pixel, a process of averaging is first executed by referring to specified numbers of pixels around the notable pixel, and then, the process of averaging is sequentially moved to a neighboring pixel.
The above-described moving average processing can exhibit an excellent effect for the noise reduction by somewhat increasing the number of referred pixels. However, to the contrary, when the number of referred pixels (the reference pixels) is increased, such a problem is encountered that resolution is degraded to produce a blurred image as a whole.
As a noise reduction method applicable not to color image signals but bi-level image signals (monochrome image signals), for example, Japanese Patent Laid-open Publication No. 37074/97 discloses a method of removing pixels in a region, which is judged as belonging to a character component, from subjecting such pixels to the moving average processing.
According to this disclosed method, with respect to a bi-level image, it is possible to remove moire contained in the image data without degrading the resolution as well as to effectively remove noise components.
Nevertheless, if the method disclosed in the above-identified Patent Publication is directly applied to color image signals, a smooth change in gradation of each color in the color image is averaged while causing a possibility of degrading the gradation of the color image.